The present invention relates generally to MR imaging and, more particularly, to a method and system to remove artifacts caused by interactions of a preparation sequence and a gradient echo sequence in MR imaging.
Typically, MR systems apply chemical shift preparation sequences commonly referred to as xe2x80x9cfat suppressionxe2x80x9d to an imaging space to suppress signals attributable to fat within an imaging subject. Reconstructing images after employing a chemical shift preparation sequence often produces a cleaner and more discernable image for diagnostic analysis. There are several known techniques of performing chemical shift imaging including a saturation technique.
With the saturation method, a frequency selective saturation pulse is applied before standard RF pulses of an imaging sequence, e.g., a spin-echo sequence. The saturation pulse operates to set the magnetization along the transverse plane of the component to be suppressed to zero. As a result, the application of the standard imaging sequence thereafter significantly reduces any signal from the suppressed component. Therefore, most signals from the suppressed component, i.e. fat, are not detected and used in final image reconstruction. A spatial saturation preparation sequence using slice selective RF pulses instead of chemically selective RF pulses may also be used to suppress spatial components of an imaging subject. Unfortunately, application of either a fat suppression sequence or a spatial saturation sequence may interact with a subsequent gradient echo sequence to generate unwanted stimulated echo artifacts.
Multi-sliced gradient echo images acquired with fat or spatial suppression preparation sequences often exhibit ghosts and/or blurring from the suppressed signal. These artifacts, which are typically generated by stimulated echoes, allow the suppressed signal from one slice to be refocused into another slice and are created through a combination of chemically selective and/or spatially selective RF pulses with a gradient echo sequence. If there is a constant phase between the suppression sequence RF pulses and the gradient echo RF pulses, then the suppressed signal appears to be blurred from slice to slice. If there is a linear phase shift from view to view between RF pulses, then the refocused signal will oftentimes appear as a severe ghost in the final reconstructed image.
Referring to FIG. 1, a schematic representation of an imaging space 2 with refocused stimulated echoes is shown. Specifically, FIG. 1 illustrates the refocusing of a signal excited by a preparation sequence from one slice into another. A protocol was prescribed such that half of the prescribed slices were outside the phantom and half the slices were inside the phantom. Prescribing slices outside the phantom provides a convenient method to observe ghosts generated by the stimulated echoes. FIG. 1 is a collection of axial images acquired with a gradient echo sequence with a spatial saturation applied through the center of the phantom. As shown, the signal excited by the intersection 4 of the preparation sequence 6 and the slice selective RF pulses 8 is refocused from one slice to another slice. This cross-talk results in severe ghosting occurring in a final reconstructed image. Ultimately, signals from stimulated echoes are refocused from one slice into another yielding a contaminated reconstructed image.
It would therefore be desirable to design a system implementing a technique that removes artifacts caused by interactions of a preparation sequence and a gradient echo sequence in MR imaging.